This invention relates to digital communications systems wherein data is serially transmitted in frames of multi-bit bytes and, more particularly, to a frame detection arrangement for use in such a system.
A relatively new transmission standard has been developed for Synchronous Optical Network (SONET), which is derived from a base rate and format, combined with a multiplexing scheme. This results in a modular family of rates and formats available for use in optical interfaces. The base rate is known as the Synchronous Transport Signal level 1 (STS-1). Each STS-1 frame is a serial bit stream of 810 bytes, with each byte including eight bits. The STS-1 framing is dedicated to the first two bytes of each STS-1 frame. The framing pattern for bytes A1 and A2 is F628 Hex (1111011000101000), where A1 equals F6 Hex (11110110) and A2 equals 28 Hex (00101000). Higher rates (STS-N) in SONET are accomplished by byte interleave multiplexing an N number of STS-1 frames. The framing pattern for an STS-3 frame in an Optical Carrier level 3 (OC-3) system is then A1A1A1A2A2A2. Similarly, the framing pattern for an STS-12 frame in an OC-12 system is twelve A1 bytes followed by twelve A2 bytes.
The basic function of a frame recovery circuit in a communications system is to recognize and lock onto the framing pattern, provide frame lock indication, and resynchronize the receiver to the incoming data stream in a minimum amount of time. It is a primary object of this invention to provide a frame recovery circuit which may be used in both an OC-3 and an OC-12 system.